This invention relates to apparatus for the road-surface-dependent control of shock absorbers of a vehicle suspension system. The apparatus includes sensors which are attached to the vehicle and which emit electrical signals which are characteristic of the undulations or unevenness of a road surface, and an evaluating circuit which receives the sensor signals and generates actuating signals for controlling the damping characteristics of the shock absorbers. The actuating signals generated are representative of the average heights of the long-wave and short-wave undulations of the road surface. The shock absorbers have at least two chambers which communicate via a controllable bypass, one of which becomes smaller during compression of the suspension system and the other of which becomes smaller during expansion of the suspension system. The controllable bypass includes throttle and/or valve devices which have respectively, a controllable throttle cross-section and/or controllable valve closing pressures which are controlled by the actuating signals produced by the evaluating circuit.
In a device described in corresponding unpublished European patent application No. 01 51 421, the road surface is scanned with an ultrasonic sensor, the electrical output signals of which correspondingly represent the road-surface undulations. The output signals are supplied in parallel to a high-pass and low-pass filter which are in each case followed by a rectifier. Accordingly, the output of the rectifier following the low-pass filter represents a measure of the long-wave unevennesses of the road surface, whereas the level of the output of the rectifier following the high-pass filter is a measure of the short-wave road-surface unevennesses. The shock absorbers of the suspension system can be switched between a soft and a hard or firm operating mode. This is carried out in dependence on whether the level of the output of the rectifier following the low-pass filter exceeds or drops below the level of a reference signal. If the level of this output is above the reference level, the shock absorbers are switched to the hard operating mode, i.e., to high damping. The output of the rectifier following the high-pass filter then influences the level of the reference signal in such a manner that the reference level is increased if the output level of this rectifier increases.
In German Offenlegungsschrift No. 2,441,172, a device is shown which operates with sensors for sensing the acceleration of the vehicle body and the distance between wheel or wheel support and vehicle body. In this arrangement, the shock absorbers are controlled in accordance with the magnitudes of this acceleration and distance, so that the actual acceleration of the body is adjusted toward a nominal value which depends on the magnitude of the distance.
In British Patent Specification No. 1,604,416, a system having hydropneumatic support units is described. These are controlled in dependence on the signals from sensors for the hydraulic pressure and sensors for the relative speed between the wheel or axle and the body. This creates the possibility of controlling the damping of the hydropneumatic support units in accordance with a predeterminable relation between pressure and speed, which is not described in detail in British Patent Specification No. 1,604,416.
German Offenlegungsschrift No. 3,426,014 describes a device having acceleration sensors at the body and at the wheel supports. The signals from these sensors are processed in a digital circuit which controls the shock absorbers of the vehicle suspension. In this arrangement, particularly low damping is achieved when the wheel and body are moving in the same direction, i.e., are subject to acceleration in the same direction. This is particularly desirable if, at the same time, the speed of the wheel axle is greater than that of the body because the movement of the body would be additionally excited in these vibration phases if the damping is too great.
An object of the present invention is to provide apparatus for the road-surface-dependent, electronically-aided control of shock absorbers of a vehicle suspension in such a way that the shock absorbers can be optimally adjusted under virtually all conceivable driving conditions, particular attention being paid to whether and to what extent vibrations are excited in the range of body and/or axle resonance by road-surface undulations.
This and other objects are attained in an apparatus which preferably includes acceleration sensors for producing signals representative of the road surface undulations, circuitry for processing these signals into control signals, and dampers or shock absorbers having a damping characteristic which can be controlled by these control signals. The control signals generated preferably include: a first signal (h) which is representative of the average height of all road surface undulations; and a second signal (w) which represents the ratio of average heights of long-wave undulations to short-wave undulations. The circuitry processes these control signals to produce at least one of a third (F) and fourth (P) type of control signal in accordance with the following relationships: EQU P=a.sub.o +a.sub.1 H+a.sub.2 w+a.sub.3 Hw EQU F=b.sub.o +b.sub.1 H+b.sub.2 w+b.sub.3 Hw
where, a.sub.o -a.sub.3 and b.sub.o -b.sub.3 are predetermined constants.
These signals are applied to controllable elements in the dampers to provide for a harder or softer damping characteristic, depending upon road and driving conditions.
This invention makes possible, with regard to small body movements, a relatively strong damping which is desirable for vibrations in the range of body resonance, and weak damping which is desirable in the case of vibrations in the range of axle resonance. If vibrations in the range of body resonance are greatly predominant, the measure of waviness w is large. If vibrations in the range of axle resonance are predominant, the waviness w has a small value. According to the present invention, the shock absorbers are adjusted to become harder or softer according to the rising or dropping level of waviness in that the throttle cross-section or the valve closing pressure, or preferably both, are adjusted to produce harder damping (narrowed throttle cross-section, increased closing pressure) or softer damping (enlarged throttle cross-section, reduced closing pressure). In this regard, a particular advantage of the invention lies in the fact that the closing pressure also depends on the product of the average height of the road-surface undulations and on the waviness, i.e., the ratio between long-wave and short-wave road-surface undulations.
To determine, on the one hand, the height of the road-surface undulations and to generate a signal representative of this height and, on the other hand, to determine the waviness and provide corresponding signals, acceleration sensors are arranged, in a preferred embodiment of the invention, at the body and at a wheel support or a wheel axle. The output signals of these sensors are additively combined with each other and with the second integral of the output signal of the acceleration sensor arranged at the wheel support or the wheel axle to generate a signal which is representative of the height of the road-surface unevenness over which the vehicle has instantaneously passed. From these output signals, which are successive in time and representative of the heights of the road-surface undulations, signals corresponding to the average height and to the waviness of the road-surface undulations are formed by the electronic circuitry.
In the preferred arrangement, the signals from the sensors are preferably exponentially averaged by the signal processing circuitry and are further processed, via a parallel high-pass filter, which passes the frequency range of the axle resonance of the vehicle, and a parallel low-pass filter, which passes the frequency range of the body resonance, to form the signal corresponding to the waviness. The output signals of the high and low-pass filters are also exponentially averaged and are then combined with each other in a dividing operation.
In this connection, it is appropriate if, for exponentially forming the mean or average, the corresponding signals are in each case supplied to a squaring stage, the output of which is connected via an input resistance to an input of a summing amplifier. The second input of this summing amplifier is feedback-connected to the output by a resistance which is slightly larger than the input resistance. The summing amplifier is followed by a root-extracting stage.
To provide the capability of rapid response to changing road-surface conditions, the acceleration sensor associated with the wheel support or wheel axle is preferably arranged close to the right-hand front wheel when driving on the right and close to the left-hand front wheel when driving on the left.
Furthermore, the acceleration sensor arranged at the body should be arranged in the front area of the vehicle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.